The present invention relates to chemical mechanical polishers used for polishing semiconductor wafers in the semiconductor fabrication industry. More particularly, the present invention relates to a new and improved slurry delivery system for delivering slurry to a chemical mechanical polisher for the polishing of semiconductor wafers.
Apparatus for polishing thin, flat semiconductor wafers are well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head, a wafer unload station, or a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically-actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is xe2x80x9cplanarizedxe2x80x9d or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in deionized water.
CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide may be formed and removed separately. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films.
Referring next to FIG. 1, a conventional CMP apparatus 50 includes a conditioning head 52, a polishing pad 56, and a slurry delivery arm 54 positioned over the polishing pad. The conditioning head 52 is mounted on a conditioning arm 58 which is extended over the top of the polishing pad 56 for making a sweeping motion across the entire surface of the polishing pad 56. The slurry delivery arm 54 is equipped with slurry dispensing nozzles 62 which are used for dispensing a slurry solution on the top surface 60 of the polishing pad 56. Surface grooves 64 are further provided in the top surface 60 to facilitate even distribution of the slurry solution and to help entrapping undesirable particles that are generated by coagulated slurry solution or any other foreign particles which have fallen on top of the polishing pad 56 during a polishing process. The surface grooves 64, while serving an important function of distributing the slurry, also presents a processing problem when the pad surface 60 gradually wears out after prolonged use.
The slurry solution is typically distributed to the slurry dispensing nozzles 62 through tubing (not illustrated), by operation of a pump (not illustrated). The force generated by the pump forcing the slurry through the tubing tends to crack the tubing, and this causes premature drying of some of the slurry in the tubing and formation of particles in the tubing before the slurry is dispensed onto the wafer. These slurry particles tend to scratch the wafer during the CMP process. Additionally, air enters the slurry through the cracked tubing, forming air bubbles which tend to adversely affect the CMP operation.
Accordingly, a slurry delivery system is needed for removing particles and air bubbles from a CMP slurry as the slurry is transported from a slurry source to a CMP dispensing nozzle or nozzles.
An object of the present invention is to provide a slurry delivery system for delivering a polishing slurry to a slurry dispensing nozzle of a chemical mechanical polisher, wherein the slurry is devoid of air bubbles when dispensed onto a wafer for polishing.
Another object of the present invention is to provide a slurry delivery system for delivering a polishing slurry to a slurry dispensing nozzle of a chemical mechanical polisher, wherein the slurry is devoid of particles when dispensed onto a wafer for polishing.
Still another object of the present invention is to provide a slurry delivery system which is capable of removing air bubbles and particles from a polishing slurry before the slurry is deposited onto a semiconductor wafer for chemical mechanical polishing of the wafer.
Yet another object of the present invention is to provide a slurry delivery system which facilitates a substantial reduction in wafer scratching during chemical mechanical polishing of the wafer.
A still further object of the present invention is to provide a slurry delivery system which optimizes the performance of a chemical mechanical polisher in the polishing of semiconductor wafers.
Another object of the present invention is to provide a slurry delivery system which may be programmed to deliver selected quantities of slurry to a chemical mechanical polisher.
Yet another object of the present invention is to provide a slurry delivery system which may be operably connected to a chemical mechanical polisher in pairs in order to provide a continuous supply of slurry to the chemical mechanical polisher.
In accordance with these and other objects and advantages, the present invention comprises a slurry delivery system which removes air bubbles and particles from a polishing slurry and delivers the slurry to a CMP apparatus for the chemical mechanical polishing of semiconductor wafers. The slurry delivery system of the present invention comprises a bag housing fitted with a slurry intake conduit and a slurry outlet conduit. An expandible and collapsible pump bag is provided in fluid communication with the conduits inside the bag housing, and the interior of the pump bag is sealed from the bag housing. As an air/vacuum controller withdraws air from the housing, the pump bag enlarges due to the negative pressure in the housing, and slurry is drawn into the pump bag through the slurry intake conduit. As the air/vacuum controller subsequently introduces air into the housing, the pump bag collapses and the slurry is expelled from the pump bag through the slurry outlet conduit. A purge valve is provided upstream of the pump bag to remove air bubbles from the slurry and vent the air to the atmosphere. A filter is provided typically in the slurry intake conduit to filter particles from the slurry before entry into the pump bag.
A pair of the slurry delivery systems of the present invention may be connected to the chemical mechanical polisher in parallel with each other, in order to provide a continuous supply of the polishing slurry to the CMP apparatus. Accordingly, as the first system undergoes the suction phase to draw slurry from the intake conduit into the pump bag, the second system undergoes the output phase to expel the slurry from the pump bag and outlet conduit to the CMP apparatus, and vice-versa. The systems may be programmed to deliver selected quantities of the slurry to the CMP apparatus.
The purge valve is located at a higher level than and upstream of the bag housing, typically at the junction between the slurry intake conduit and the bag housing, to facilitate the destruction of air bubbles in the slurry as the air bubbles rise in the slurry from the intake conduit into the purge valve. In a preferred embodiment of the invention, the purge valve includes a rotation floater which is rotatably mounted in a purge valve housing. A spring-loaded valve ball is slidably disposed in the purge valve housing above the rotation floater. During the suction phase of the pump bag, the rotation floater engages a floater support and the valve ball engages a ball stop shoulder in the purge valve housing to prevent flow of slurry out of the slurry intake conduit and into the purge valve. During the output phase of the pump bag, the rotation floater disengages the floater support and the valve ball disengages the ball stop shoulder. Accordingly, as slurry flows into the purge valve housing and past the rotation floater, the rotation floater rotates and destroys air bubbles in the slurry. The air from the broken air bubbles rises beyond the valve stop shoulder and valve ball and is vented from the system through the vent port.